Heat the vegetable oil in a large saucepan over medium-high heat. Add the mustard seeds, curry leaves, peanuts and chana dal; when the mustard seeds start to crackle and toast, add the green beans and saute until they are almost tender and bright green, about 4 minutes.
Add the lime juice, jalapeno, cilantro and a pinch of salt; cook 1 more minute. Add the coconut and stir until warmed through. Discard the curry leaves. Taste and adjust the seasoning with salt. Serve hot or refrigerate and serve cold.
Chana dal, or dried split chickpeas, gives this dish a nutty flavor and crunch.